CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon), which have been heretofore used as a refrigerant for refrigerating equipment, are now subject to regulation from the recent fear of ozone layer destruction, and alternatively HFC (hydrofluorocarbon) has come to be used as a refrigerant. However, among HFC refrigerants, HFC-134a, which has been used for a car air-conditioner refrigerant like a standard material, is also subject to regulation in Europe, because its global warming potential (GWP) is high, despite its zero ozone depletion potential (ODP).
Under such circumstances, there is an urgent need to develop a refrigerant that has little influence on the ozone layer and exhibits low GWP. For example, Patent Literature 1 discloses a (Z)-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz) refrigerant as a refrigerant that has little influence on the ozone layer and exhibits low GWP.
In the case of a conventional refrigerant such as CFC and HCFC, a refrigerating machine oil containing a hydrocarbon oil, such as a mineral oil and an alkylbenzene, has been used favorably, however, since a refrigerating machine oil may exhibit, depending on the kind of a coexisting refrigerant, an unpredictable behavior in terms of compatibility with a refrigerant, lubricity, viscosity of a solution with a refrigerant, and thermal and chemical stability, development of a refrigerating machine oil for each refrigerant becomes necessary.